A diesel particulate filter (hereinafter referred to as “DPF”), for example, is known as a filter for collecting particulate matter (hereinafter referred to as “PM”) contained in exhaust gas emitted from a diesel engine.
The DPF can only collect a limited amount of PM. Thus, so-called forced regenerations need to be performed, in which accumulated PM is periodically burned and removed. In a forced regeneration, unburned hydrocarbons (HC) are supplied to a diesel oxidation catalyst (hereinafter referred to as “DOC”) on an upstream side in an exhaust gas flowing direction through in-pipe injection (fuel injection into an exhaust pipe) or post-injection to cause oxidation, and to raise the temperature of the exhaust gas to a PM burning temperature.
Known techniques for detecting an amount of accumulated PM collected by a DPF include, for example, a technique of estimating the amount on the basis of a pressure difference across the DPF and a technique of estimating the amount from the electrostatic capacity (capacitance) between electrodes provided in the DPF (e.g., see PATENT LITERATURE DOCUMENTS 1 and 2).